A Theoretical Study Of Subthreshold Odd-even Harmonic Radiation From Asymmetric Molecules

In recent years,the research of high-order harmonic generation(HHG)in strong laser-matter interaction has attracted a great deal of attention.Many efforts have been devoted to this issue and important theoretical and experimental progresses have been made.In general,the high order harmonic spectrum has three parts,the near-threshold region,the platform region and the cutoff region.In this paper,near-threshold harmonic generation of asymmetric molecules including odd and even harmonic orders in strong laser fields is studied by solving the time-dependent Schrodinger equation numerically.The first chapter briefly introduces some basic physical phenomena in the process of interaction between laser field and matter,such as strong field ionization and HHG,and introduces the application prospect of HHG.The second chapter starts with the introduction of the emission mechanism of HHG,then introduces some theoretical methods for studying HHG,such as strong-field approximations,classical trajectory Monte-Carlo model and spectral method.Here,the mechanism of odd-even HHG from asymmetric molecules and some key physical quantities in the analysis of HHG such as the ellipticity of HHG are also discussed.In chapter 3,based on the numerical solution of time-dependent Schrodinger equation,below-threshold odd-even harmonic emission of asymmetric molecules in strong laser fields with different laser parameters is explored.The ellipticity of odd-even HHG under different laser wavelengths and different orientation angles(the angle between the molecular axis and the laser polarization direction)is analyzed,and the generation mechanism of near-threshold odd-even harmonic is discussed.It is revealed that the excited state effect,the two-center interference effect,the resonance effect,and the interference between different transition paths play important roles in the emission of near-threshold odd-even harmonics of asymmetric molecules and remarkably influence the ellipticity of these harmonics.Because odd versus even harmonics respond differently to the effects mentioned above,odd and even harmonics at different harmonic orders show different orientation dependence.